This is the sound of insect eating bats hunting for insects in the dark as recorded from my back-yard. The bat noises start at 10 kHz and up, so I have filtered all frequencies below that to lose the 'rumble' of the city.Recording chain: Audio Technica AT-3032 (microphones 120 degree Y config) - Sound Devices Mix Pre (Mic preamp) - Edirol R-09 (16 bit 44.1 kHz).